Investment casting by the "lost wax" process was first known to be used by the Egyptians and still finds practical utility today. The process involves many steps which may be summarized as follows:
a. Mold, carve, or machine a wax pattern.
b. Dip the wax pattern in a series of ceramic slurries in order to create a ceramic shell which is allowed to dry somewhat around the wax pattern.
c. Heat the ceramic shell and wax pattern in an oven or an autoclave until the wax melts and evacuates the ceramic shell.
d. Fire the ceramic shell.
e. Pour a melted alloy into the ceramic shell and allow it to solidify.
f. Break the ceramic shell to obtain the solidified alloy in the shape of the wax pattern.
Using investment casting techniques, parts may be molded having shapes that could not be molded by other techniques. Investment casting also offers the potential for manufacture of parts made from many different alloys.
Currently, some of the problems involved with the investment casting process particularly related to the wax pattern are as follows:
a. Production of a wax pattern is time consuming if made by conventional machining techniques, and if the wax pattern is molded, shrinkage of the wax upon cooling creates loss of tolerances due to uneven solidification rates as it cools to a solid.
b. The wax is sometimes brittle and easily damaged before and during the ceramic slurry coating process. In addition the drying of the ceramic shell must be conducted under tightly controlled temperature conditions, since an increase in temperature of the wax pattern during the shell drying process may crack the shell due to pattern expansion. Also the ceramic slurry must easily wet the wax pattern to form a good mold that faithfully represents the pattern's shape and surface finish.
c. During the wax pattern removal stage, the wax must melt in such a manner as to not cause cracking of the mold due to the expansion of the pattern as it heats up.
d. During the ceramic shell firing, any wax that remains or has soaked into the shell must burn-off leaving very little residual ash.
Solid Imaging, or the direct production of objects or mold patterns from computer aided design data, holds promise to solve several of the problems mentioned above relative to use of wax patterns for investment casting. Many patents have issued in the field of solid imaging that describe the technologies' potential use in investment casting, but prior to the invention of a photoformable composition as described herein, no photoformable composition had been developed that would solve in particular the above described shell cracking problem.
In a patent assigned to DeSoto, Inc., Des Plaines, Ill. (U.S. Pat. No. 4,844,144, Jul. 4, 1989) there is proposed the use of a photosensitive ethylenically unsaturated liquid which solidifies to a thermoset shape upon exposure, in mixture with primarily a substantially inert low temperature thermoplastic material which becomes partially solidified or somewhat bound within the photosolidified thermoset matrix. In this patent and in a publication "Investment Casting of Optical Fabrication and Stereolithography Models" by Myron J. Bezdicek of DeSoto (published by the Investment Casting Institute, 1989, 37th Annual Technical Meeting) it is proposed that the thermoplastic component of the photopolymer composition softens during melt-out or burn-out of the photoformed pattern and thereby weakens the thermoset structure of the pattern lessening the effects of expansion during subsequent heating. Although this approach is believed to provide an improvement when compared to the use of other photosensitive compositions to produce patterns for investment casting purposes, mold cracking still occurs and further improvements are necessary. The author and inventor propose other fixes such as block or flask casting, wax coating, and hollow-walled patterns, but for other practical reasons these fixes are not always desirable approaches for the investment caster.
U.K. Patent Application GB 2207 682 A (published Jun. 8, 1989) discloses a photosensitive composition, for use as a pattern for dental braces, in which one component of the pattern softens prior to burn-out of the pattern thereby preventing the build up of pressures that might cause shell cracking.
A patent application in Japan, which is a "laid open to public unexamined application" 2-116537 (Matsushita Electric Works), with a publication date of May 1, 1990, describes the advantages of using expandable microspheres, balloons, and/or rubber bead fillers, within a photocomposition to prevent shrinkage and the build-up of stresses during solid object formation by essentially solid imaging means. Although the compositions described, specifically in the relation to the addition of Expancel.RTM. microspheres to the composition, have similarities, the application does not discuss the use of these compositions for the production of investment casting patterns, and the applicants apparently were unaware of the rather surprising fact that the microspheres and/or balloons could be thermally collapsed in addition to being thermally expanded. In addition, the application describes only ultrasound methods of keeping particles uniformly in a dispersion.
Other pertinent art is disclosed in U.S. Pat. No. 3,822,138 (Norguchi et al.) in which carbon microspheres are added to a wax pattern composition in order to reduce pattern shrinkage during pattern composition cooling in the mold. Although, such carbon microspheres may decrease pattern shrinkage during molding, they are not added to allow thermal collapse and reduction of pressures during pattern melt-out or pattern burn-out steps in an investment casting process as disclosed herein by the instant invention.
A published unexamined Japanese Application Kokai: 50-7722 (Published Jan. 27, 1975) describes a method by which surface blisters in a molded wax pattern can be reduced by mixing phenol-based or vinylidene chloridebase fine hollow resin spheres in the wax prior to molding. The publication describes excellent ways to mix and prepare the wax/hollow sphere compositions, however, the publication does not describe any advantages relative to the reduction of shell cracking during the melt-out and firing stages of mold production. Typically the phenol-based microspheres are thermoset and therefore not thermally-collapsible as described in this disclosure. And typically the vinylidene-chloride based microspheres, which generally have a glass transition temperature on the order of -15.degree. C. have higher diffusion rates at room temperature. This greater diffusion rate would allow the blowing agent used to create the microsphere to escape, causing premature collapse of the spheres or replacement of the blowing agent with other gasses. The production of copolymer vinylidene-chloride and acrylonitrile hollow microspheres, which have lower diffusion rates at room temperature, was very difficult in 1973 when the above Japanese application was filed. However, in February 1978 the Dow Chemical Company disclosed (U.S. Pat. No. 4,075,138, J. L. Garner) a method of making copolymer vinylidene-chloride and acrylonitrile hollow microspheres, which contained an isobutane blowing agent, in production quantities. It is these, or very similar, microspheres, herein disclosed, that are comprised in the pattern compositions of this invention and that aid in the reduction of shell mold cracking during the investment casting process.
Other pertinent art is disclosed in U.S. Pat. No. 4,790,367 (Moll et al.) wherein a foamed pattern is produced, coated with a silica slurry, packed in sand, and burned out using molten metal. In essence the process, which is termed a "Lost Foam" or "Lost Plastic" process, consists of the following steps:
1. Prepare the Plastic Material: By preparation of the plastic material, beads, typically consisting of PMMA or other resins containing an unexpanded blowing agent, are produced.
2. Pre-expand the beads to a loose packed density of around 10% greater than used in the molded form.
3. Age the beads. This usually involves drying the pre-expanded beads.
4. Mold the beads. In this step the beads are pneumatically loaded into the mold and steamed, causing them to bond together into a foamed pattern.
5. Age the molded pattern. This typically involves drying the pattern.
6. Assemble the pattern parts.
7. Refractory coat the pattern. The refractory coating provides a smoother finish than would be given by the packed sand (from step 9 below) and helps contain the molten metal during the casting step.
8. Attach the gates, runners and sprues.
9. Pack the pattern, gates, runners and sprues with sand, which is vibrated to create greater compaction.
10. Pour the casting. In this step, the foam resins are effectively thermally degraded to lower molecular weight volatile components which are essentially burned out of the packed space and replaced by the molten metal. This disclosure describes methods and compositions to reduce carbon residue during this burn-out process.
11. Cool the casting.
12. Remove the sand from around the casting.
13. Clean the casting.
U.S. Pat. No. 3,942,583 (Baur) discloses a similar "Lost Plastic" process in which foam pieces, such as foam plates, foam beams, foam columns, etc., are assembled together, then sand packed, and burned out by the pouring of the molten metal. Also U.S. Pat. Nos. 4,773,466 (Cannarsa et al.) and 4,763,715 (Cannarsa et al.) disclose a "Lost Foam" process utilizing alternative resins to form the foam beads. Although such art involves heat destruction of foamed patterns, the art does not disclose the reduction of shell mold cracking in an investment casting application utilizing thermally-collapsible microspheres mixed in a pattern composition.
Another U.S. Pat. No. 4,854,368 (Vezirian) discloses the added step of applying a vacuum during heat removal of a foamed pattern prior to filling the invested mold with metal. The added vacuum prevents decomposition products from entering the atmosphere. Other disclosures, such as U.S. Pat. No. 4,891,876 (Freeman), U.S. Pat. No. 4,830,085 (Cleary et al. ), U.S. Pat. No. 4,787,434 (Cleary et al.), U.S. Pat. No. 4,520,858 (Ryntz, Jr. et al.), and U.S. Pat. No. 3,426,834 (L. J. Jacobs et al.) describe various other modes and methods of conducting "Lost Foam" casting to make various objects.
In a patent assigned to Spectra-Physics, Inc., San Jose, Calif. (U.S. Pat. No. 4,915,757 Apr. 10, 1990) there is described a process whereby small balls, microspheres, grains of sand, etc., initially bonded together by a wax or a weak cement within a block, are removed in selective regions by laser heating or blasts of impinging hot air in order to form three-dimensional models. No specific mention is made as to the utility of these models for investment casting purposes, and indeed the material of the balls does not appear to make the models produced by this method useful for investment casting applications, since there does not appear to be consideration of the thermal expansion characteristics required and the burn-out characteristics necessary relative to the materials proposed.
Also published by the Investment Casting Institute, 1989 at the 37th Annual Technical Meeting is a paper "Applications of Stereolithography in Investment Casting" by Frost R. Prioleau, of Plynetics Corporation, in which a "proprietary process" using stereolithography patterns for investment casting is mentioned. This process apparently reduces cracking of most shells, however, the need for new photopolymers with better "removability" is also stated.
An excellent insight into "The Causes and Prevention of Shell Mould Cracking" a materials and processes committee report is published by the British Investment Casting Trade Association (August 1975, Royton House, George Road, Edgbaston, Birmingham, B15, 1 NU). This publication describes the effects of pattern wax on the mold shell during the various production steps and especially suggests that during the wax pattern melt-out from the ceramic shell mold, waxes should be chosen that have low thermal conductivity and high permeability into the ceramic shell. The thinking is that during the melt-out step, heat is transferred from the ceramic shell to the outer layer of the wax pattern causing the outer layer to melt. If just the outer wax layer melts first, without a substantial amount of heat being conducted into the inner region of the wax pattern, which would cause expansion and shell cracking, and if the outer wax layer can flow out of or permeate into the ceramic shell quickly enough, then shell cracking will be minimized.
It may be that shell mold cracking using the process suggested by DeSoto still occurs since, although the thermoplastic component of their patterns softens, there is still substantial resistance to flow or escape of this melted component due to constraint within the photo-thermoset matrix.